1. Field of the Invention
The present invention relates to a high frequency amplifier, and more specifically to a high frequency amplifier having a desired band width, for use in a tuner circuit for an optical communication system, radio communication system, a television receiver, and the like.
2. Description of Related Art
With advance of optical transmission technology, research is now being conducted for a high speed optical transmission system using an optical device of a long wave band and a single mode fiber, which enable a large amount of long distance transmission. In particular, in order to realize a wide band information communication network for giving a variety of communication services including images, data and audio, it is desired to speed up and stabilize the above mentioned optical transmission system. In addition, a basic transmission line or truck used in this wide band information communication network is required to have a channel capacity of a few gigabits per second in the case of a time division multiplexed transmission line, and optical transmitter/receiver apparatuses included in the optical transmission system are required to have a wide band and a high speed operation.
A fundamental function, which should be possessed by the optical transmitter/receiver apparatuses, can be basically divided into three "R"s, namely, a reshaping by an equalizing amplification, a retiming and a regeneration. Here, the retiming means to a timing signal extraction function for extracting from a received data signal a timing signal in synchronism with a transmission channel data rate, and for supplying the extracted timing signal to an identification circuit and a multiplexing terminal.
In tuners used in mobile communication instruments such as portable telephone and in television receivers included in domestic electronic instruments, on the other hand, there is a large demand for miniaturization, low power consumption and low cost. Under this circumstance, efforts are being made for implementing a high frequency circuit for these instruments on an integrated circuit.
In the optical transmitter/receiver apparatuses and tuners as mentioned above, it is a matter of course that a wide band amplification circuit is significant as one constituent, but amplification circuits having a relative narrow band width, used in an intermediate frequency circuit, for amplifying only a signal within a desired band width, are also a significant constituent.
Referring to FIG. 1, there is shown an example of an ordinary conventional high frequency circuit used in a timing extracting circuit of the above mentioned optical transmitter/receiver apparatuses. The shown high frequency circuit uses GaAs field effect transistors Q.sub.101 and Q.sub.102 for the purpose of obtaining a signal amplification characteristics in a desired band width, and between the transistors Q.sub.101 and Q.sub.102 there are connected a bandpass filter 101 which has the above mentioned desired band width and composed of a concentrated constant circuit or lumped element circuit including inductors and capacitors. For example, this conventional high frequency amplifier has a characteristic having a band width of 1 GHz in a 5 GHz band, namely, Q=5.
Turning to FIG. 2, there is shown a second example of the conventional high frequency amplifier, in which an operating frequency band is made higher than that of an amplifier by using a microstrip line formed on a thin film (hybrid) integrated circuit. This conventional high frequency amplifier includes an impedance matching stub circuit 201 which is located at an input side of a transistor Q.sub.201 for amplification and which is formed of a microstrip line. In this conventional circuit, since the impedance matching circuit is designed to match with a frequency characteristics of the transistor Q.sub.201, it is possible to easily realize a high frequency amplifier having a desired band characteristics, within a frequency zone of a 10 GHz band.
In addition, referring to FIG. 3, there is shown a third example of the conventional high frequency amplifier, which is disclosed in Japanese Utility Model Post-examination Publication No. Sho 62-41448. This conventional high frequency amplifier includes amplifying transistors Q.sub.301 and Q.sub.302 having a waveguide matching circuit at an input side and at an output side, respectively, and a filter 301 formed of a constant-resistance bridged-T circuit for compensating an amplitude in a desired band. This circuit is characterized in that, by utilizing a constant resistance property of the bridged-T circuit having the desired band, the amplifier is constituted in a conjugate matching condition viewed from the output terminal of the transistor Q.sub.302. With this feature, a flat frequency characteristic can be obtained while maintaining a low noise property and a linearity.
However, the first conventional example as mentioned above using the lumped element circuit, is disadvantageous in that a realizable frequency is on the order of 5 GHz, because the operating frequency is restricted by influence of floating elements.
In the second embodiment using the microstrip line formed on the hybrid integrated circuit, and in the third embodiment using the waveguide circuit elements, the realizable frequency can be elevated to 10 GHz. However, since it is necessary to design the impedance matching circuit by taking into consideration the frequency characteristics of the transistors for the purpose of realizing the desired frequency characteristics, it is impossible to individually design each of the amplification section and the bandpass filter section, independently of the others. Therefore, the design becomes a single-purpose design, and accordingly, can have neither a general-purpose design nor a flexibility in design.